Stars Eating Planets

Abstract

In a phase of instability during the dynamical evolution of some multiple exoplanetary systems, exchange of angular momentum and/or energy through gravitational interactions between planets will lead to alterations of their orbital properties. In some cases, planetary orbits end up crossing, which leads to close encounters between two planets. In turn, they undergo a planet--planet scattering event, where a large exchange of angular momentum and energy leads to a significant change in trajectories and thereby their orbits. The distribution of eccentricity for observed giant planets provides strong evidence for past planet--planet scattering events in many exoplanetary systems. One particular outcome of such an event is when one of the planets ends up on a highly eccentric orbit, which leads to a planet--star collision, also referred to as planet consumption. When a planet is consumed, it can transfer physical quantities such as angular momentum, energy and heavy elements to the host star, altering its properties. Due to the transfer of said quantities, a planet consumption event can have observational consequences for the host star that are detectable by astronomical instruments.

In this thesis, I first employ a semi-analytical two-body model to constrain which type of orbital configuration in a planetary system with a single star will facilitate planet consumption. I then use the constrained parameter space to formulate a fiducial planetary configuration. The dynamical evolution of said system is then modelled using 100 numerical N-body integrations, which allows me to further determine for which type of systems that planet consumption by planet--planet scattering is possible. Moreover, I tie the results of the semi-analytical and numerical analysis to a literature study in order to constrain which type of observational consequence will dominate for consumption of planets on highly eccentric orbits.

From the two-body analysis, I conclude that planet consumption is more probable for scattering events where: the planetary mass ratio is extreme, where the inner planet is less massive than the outer; the planets are orbiting a host star with low density; at least one of the orbits is highly eccentric, preferably that of the least massive planet; the event occurs at small separations from the host star. Based on these results, I also formulate the fiducial planetary system, which is a Solar System analogue with two Earth-mass planets inside of 1 AU and three initially unstable Jupiter-mass planets beyond 5 AU. From the numerical N-body integrations I find that hierarchical systems with low-mass planets and at least two unstable giant planets will consistently induce consumption of planets of 30 Earth masses and less. When the system has three giant planets, around 10% of the integrations lead to the consumption of a giant planet. Such an event can produce an observational consequence where a single giant planet ends up on a very distant orbit with arbitrary eccentricity. The integration results also show that there are three extreme pathways to planet consumption: diffusive planet consumption, where the eccentricity of a planet increases diffusively over a large number of scattering events; strong planet consumption, where the eccentricity is boosted up quickly over a small number of scatterings and Lidov--Kozai planet consumption, where the eccentricity increases through the Lidov--Kozai mechanism which excludes planet--planet scattering.

In the literature study I determine that the dominant observational consequence highly depends on stellar properties such as age, metallicity, mass and radius, as well as planetary mass, radius and composition. Moreover, the minimum separation between the planet and the host star during an orbit determines the strength of detectable signatures. A majority of the observational consequences are difficult to directly tie to planet consumptions, meaning that detections of such events are good targets for future multi-waveband astronomy missions. From the results of the numerical integrations performed, I estimate that the dominant observational consequence from planet consumption in the Milky Way is metallicity enhancement by consumption of super-Earths. Outside the Galaxy, the dominant observational consequence is planet merger transients caused by the consumption of a giant planet, which induces an increase of stellar luminosity in the optical/infrared wavebands followed by a radio afterglow that lasts for a few thousand years.

Popular Abstract

Astronomers make for excellent investigators and the Milky Way provides an exciting crime scene full of clues and evidence of remarkable events in the past. Stars have collided with one another. Stars have been swallowed by black holes. Stars have exploded in supernovae, leaving black holes or neutron stars behind. Our galaxy has had a violent past, to say the least. But how do we know? Well, compared to detectives investigating criminal cases back on Earth, astronomers have a significant advantage. They can look back in time. The light shining down on us from countless of stars on the night sky during an evening stroll holds many secrets. Not only can the light tell us about stellar ages, masses, sizes and compositions, it can also provide us with clues about destructive events that have occurred in their past. For exoplanetary scientists, there is one mysterious case in particular that needs solving: Where have all the planets gone?

Observations of exoplanetary systems indicate that the Solar System is special. The large number of single planet systems and the lack of planets on short-period orbits near the Sun or highly elliptical orbits are all observational signatures that thicken the plot. Moreover, according to planet formation theory, planets are likely to be born on circular orbits in systems with more than one planet. In turn, there must be some mechanism that eliminates planets from the systems between their birth and the time of observation, while putting remaining planets on short-period and/or elliptical orbits. The clue that blows the case wide open is the fact that observed planetary systems with multiple planets have large average separations, meaning that there is a lack of systems where the planets are tightly packed together. Computer simulations of the evolution of planetary systems where the planets have small initial separations show that they are highly unstable. This instability often resolves in a violent celestial dance where the planets gravitationally interact and alter each other's orbits. This is called planet--planet scattering.

One particular outcome of this chaotic dance is that a planet can end up colliding with its host star in an event referred to as planet consumption. Depending on when and where the planet hits the host star and its impact velocity, it can alter different properties of the host star. A faster rotational velocity around its axis than predicted by theoretical models, a notable increase in metals within a star's atmosphere or a brief increase in its brightness are all smoking guns that point towards a past planet consumption event.

In this thesis, I have used an analytical model and computer simulations to investigate which type of planetary system architectures that consistently produce planet consumption events by planet--planet scattering and what kind of planet is more likely to end up being eaten. I have then performed a literature study of observational consequences induced by planet--star collisions to couple my simulation results to detectable signatures in the properties of a host star. I found that planets with thirty times the mass of the Earth and less consistently get eaten when they are in the same system as at least two giant planets of a third of Jupiter's mass or more. The low-mass planets are likely to create signatures in the light from the culprit star showing an increase in Lithium and heavy metals. A giant planet can also get eaten in 10% of the systems if they are born with at least three giants. This can produce a large variety of observational signatures that can be seen millions of light-years away in neighbouring galaxies such as Andromeda. I found a previously unexplored observational consequence where the consumption of a giant planet can lead to the existence of systems where a single giant planet orbits the host star at large distances. The existence of such systems is poorly explained by planetary formation theory.

While the signatures caused by planet consumption generally are weak, the next generation of astronomical instruments such as the Square Kilometre Array and the Vera Rubin Observatory will most probably be able to consistently detect such events. In turn, exoplanetary scientists will develop their understanding of the early evolution of planetary systems and obtain a major piece of evidence regarding the mysterious uniqueness of the Solar System.

• author: Wimarsson, John
• supervisor: Alexander Mustill
• organization: Lund Observatory - Undergoing reorganization
• course: ASTM31 20201
• year: 2020
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Earth and planetary astrophysics, planetary dynamics, planet-planet scattering, planet consumption
• publication/series: Lund Observatory Examensarbeten
• report number: 2020-EXA171
• language: English
• id: 9032623
• date added to LUP: 2020-12-09 09:23:35
• date last changed: 2020-12-09 09:23:35